Delayed coking is a process that is commonly used in the petroleum refining industry for converting or upgrading heavy residual oils to lighter distillate products and coke. In delayed coking, the heavy residual oil is initially heated in a furnace to a temperature at which thermal cracking begins. In the “charging” phase of the process, the heated feed is directed from the furnace into a large coking drum, whereupon the cracking proceeds over an extended period of time. The cracking process results in the production of hydrocarbons that are lighter (i.e., have a lower molecular weight than the feed) and are in vapor form. These vapors rise to the top of the coking drum and are led off to a downstream product recovery unit. During the thermal cracking of the feed, coke is also produced and is gradually accumulated inside of the coking drum. Once the level of coke within the coking drum has reached a predetermined limit, the introduction of the new feed into the coking drum ceases. Any vaporous products that remain in the coking drum at that point are purged from the coking drum using steam. After that purging process, the built-up coke is quenched with water. The coke inside the coking drum is then broken up, typically using hydraulic jetting or cutting with high pressure water jets. The lower end of the coking drum is then opened and the broken-up coke is discharged from the coking drum via a bottom chute. At that point, the coking drum and its various components may be further processed (e.g., rinsed), etc. and the delayed coking process will be repeated.
With initial reference to FIG. 1, a portion of a typical coking drum 100 (or simply “drum”) of the type used in the delayed coking process is shown. The drum 100 is a large vessel that is often 3-10+m in diameter and 10-30+m tall. The drum 100 includes an outer shell that is typically made of unlined or clad steel and that ranges from about 10 to 30 mm thick. The outer shell includes an upper cylinder portion 102 and a lower frusto-conical portion 104 that terminates in a lower cylindrical section 105 having a smaller diameter. The cylindrical section 105 of the lower portion 104 of the shell is typically closed off by a bottom closure disk 106 or, alternatively, a mechanical valve. During the discharge process discussed previously, this closure disc 106 is opened to enable broken-up coke to flow out of the drum 100. The drum 100 is often supported over a ground surface G by a support skirt 108 that is mounted to a lower exterior portion of the shell. In this particular case, as illustrated in the detail portion of FIG. 1, a weld 110 joins the top end of the support skirt 108 with the bottom of the upper cylindrical portion 102 of the drum shell.
In most delayed coking operations, coking drums operate together in pairs and in alternating fashion in order to provide a semi-batch process. Each pair of coking drums sequentially proceeds through the charge-quench-discharge cycle outlined above. Thus, while one coking drum is being charged with heated feed, the other is quenched and discharged in a semi-continuous process. This results in each coking drum being heated and cooled repeatedly. This repeated heating and cooling causes high metal stresses to develop in the area of the junction between the drum 100 and its support skirt 108. This occurs, for example, when quench water is introduced into the drum to quench the coke. When the quench water is introduced, the drum exterior is much hotter than the quench water inside the drum, and the temperature differential between the drum interior and the drum exterior results in large thermal gradients. These thermal gradients cause high metal stresses. As a result, the skirt-to-shell junction weld 110 and adjacent areas are susceptible to fatigue failure due to the severe thermal-mechanical cyclic stresses. Cracks often develop in the area where the support skirt 108 is attached to the drum 100. In certain severe cases, the drum 100 separates entirely from the support skirt 108 as a result of these cracks, resulting in a very dangerous condition.
One early industry practice for addressing this problem was to reduce the local stiffness and stresses close to the skirt-to-shell junction weld 110. This could be accomplished, for example, by placing simple vertical slots in the support skirt 108. The earliest version of these vertical slots had squared-off ends that terminated without any special geometry and that were simple to machine. These slots were found to provide a modest improvement to the life and performance of the skirt attachment. Later, as depicted in FIGS. 2 and 3, these simple slots were replaced with slots 112 having a larger diameter hole (also called a keyhole 114) at each end of the slot. These slots 112 and keyholes 114 were found to further reduce the local stiffness and stresses close to the skirt-to-shell junction weld 110. However, both of these methods have had only moderate success. In one example, the results from a finite element analysis of a support skirt having this type of traditional slot indicated a useful life of less than 1.8 years. While the useful life in other cases will vary, a longer useful life than that provided by conventional slots 112 and the like is desired.
Accordingly, what is needed, is a method and apparatus that improves the resistance to fatigue cracking due to the thermal-mechanical cyclic stresses in a skirt-to-shell junction of a coking drum.